Microscope objectives typically are corrected in such a manner that the objects at the boundary layer to the immersion medium are imaged perfectly. Spatially expanded objects, which are not located at this boundary layer, can be captured by moving the microscope objective in the direction of the object. If the embedding medium and the immersion medium have the same refractive index, this imaging process also will yield a perfect image. But if the refractive indices are not identical, which usually is the case, a spherical aberration will occur, which significantly impacts the image quality.
One example for this is the utilization of a 100×/1.4 objective with an oil immersion with a refractive index of 1.518 and an embedding medium with a refractive index of 1.456. If the object is placed directly under a cover slip, it will be imaged perfectly. But if the object is located at a distance of 0.02 mm from the cover slip, the imaging is achieved by moving the objective 0.02205 mm in the direction of the object. However, this image will be of very poor quality due to the great spherical aberration.
Another example is a 20×/0.8 objective in air and an object in water with a refractive index of 1.334. If the object is located at the water surface it will be imaged perfectly.
For an object position of 0.06 mm below the water surface, the objective must be moved 0.0405 mm in the direction of the object. This image also will be of very poor quality due to the spherical aberration.
For a third example, a 40×/1.2 objective is used with a water immersion and an object in water. If the cover slip has the required thickness of 0.17 mm, the object will be imaged perfectly. But if the cover glass thickness deviates from the required target value by 0.04 mm, the objective must be moved 0.032 mm in the direction of the object. This image again will be of very poor quality due to the spherical aberration.
Typically, the problem described above will be solved with the use of correcting objectives. These are objectives with moveable subassemblies of lenses, which are moved along the optical axis. These objectives are designed in such a way that moving these lens assemblies creates spherical aberration. This spherical aberration then compensates for the spherical aberration mentioned in the previous section.
Correcting objectives usually are very complex. In order to facilitate the functionality of creating a specific spherical aberration, additional lens assemblies must be present in the objective. That is why correcting objectives are very cost-intensive. As it is near impossible to move lens assemblies without some mechanical play, correcting objectives often suffer from coma and chromatic errors.
Other solutions are based on the principle of reproducing the image created by the microscope with the use of a complex optical system. Inside this additional system, the correction of the objectionable spherical aberration is performed with the use of moveable lens assemblies or via deformable mirrors.